The basic elements of an installation-type conferencing system are now described in connection with FIG. 1. The main element of the system is equipment 100, which generally processes incoming and outgoing signals, and may also include certain control and sound quality functions, some of which are described below. This type of conferencing system is referred to as “installation-type,” as equipment 100 is intended to be fixed in place in the local environment, as opposed to certain portable conferencing system devices such as the CHAT® 50 or the CHAT® 150 available from ClearOne Communications, Inc., of Salt Lake City, Utah, which can be moved from room to room or from tabletop to tabletop without the installation of cables and wires within a room. One or more local microphones, 104a to 104n, are normally installed within the room environment to pick up the speech of room participants, the signals of which are routed to equipment 100. Equipment 100 contains electronics or intelligence to combine the local microphones 104a to 104n as appropriate, for example, by switching between them or by the mixing of signals.
Certain signals present in equipment 100 are intended to be delivered to local participants. To this end, equipment 100 is connected to a public address and/or amplification system 102 for delivery to one or more speakers 106a to 106n. These speakers 106a to 106n are often mounted in the ceiling or high on a wall in the room in which conferences are to take place. Note that amplification system 102 may not necessarily be distinct from equipment 100, but, rather, equipment 100 may include such amplification circuitry if that is desired. However, a separate amplification system 102 is often used, such that audio from other sources 110 may be produced at the speakers 106a to 106n, for example, from audio-visual equipment, intercom or public address systems.
Those elements alone are sufficient to fashion a local conferencing system in which all of the participants are located in the same room, and it will become clear that certain of the inventions described herein are applicable thereto. However, it is often the case that a conference will involve distant participants that are not located within the room environment of the conferencing system. Therefore, equipment 100 may also include one or more incoming ports 108a to 108n and one or more outgoing ports 109a to 109n. Note that incoming and outgoing ports are usually paired, so that a distant participant can both speak and hear a conference.
A conference, as the term is used herein, may occur in many environments and is not restricted to the traditional definition. Herein, a conference is a meeting between participants utilizing a system that produces room-audio that is exposed to microphones connected to that system. So, for example, a conference may occur in a classroom 120 setting, such as that shown in FIG. 2A. Here, a number of student stations (including a chair and a desk) are arranged with a view of video monitors 124. The system permits students to interact with a classroom lecture or discussion through microphones 122 built into the desk surfaces. An audio system, not shown, produces audible sound into the room, which is both heard by the room participants and picked up by microphones 122.
FIG. 2B depicts a more traditional conferencing environment, a boardroom 126, with multiple microphones 122 for local participant use and monitors 124 with which distant participants may be seen, if a video feed is provided. Here, wall-mounted speakers 123 are provided and connected to the conferencing equipment for the production of sound into the boardroom 126 generally.
A conferencing system, however, may vary from these conceptual peer-based models. For example, as illustrated in FIG. 2C, a conferencing system may be incorporated into a courtroom 128 which may include traditional public address and/or amplification functions. Microphones 122 may be provided for a central speaker at a podium, at the tables at which counsel may be seated, at the judge's bench, at the witness stand, etc. Traditionally, all the participants to a hearing or trial would be physically present in the courtroom 128. However, courts have begun to recognize that a participant may be “present” as to a proceeding but not be physically present in the courtroom 128 through the use of audio and audio/video links. For example, a hearing on a routine motion might be conducted with one or more of the parties' counsel present over the telephone. In another example, it may be desirable to avoid exposing a witness to the pressures of a courtroom 128, for example, a child witness. Regardless, the audio of the participants will be produced in the courtroom 128 environment and picked up by the microphones 122.
Because of the coupling between the speakers and microphones in a room, a certain undesirable effect is introduced, hereinafter called “echo,” which will now be discussed in connection with FIG. 3. Echo is produced in a system 200 where a distant participant is involved. Here, a distant participant is included in a conference through a carrier medium 212, which may be, for example, a telephone line, an Internet connection or a long-distance radio or satellite connection. The audio from a distant participant is produced at speaker 202, carried through path 214, picked up at microphone 204, and delivered again to the distant participant who hears an echo of himself with a delay equal to two times the propagation delay over the carrier medium 212 plus the audio delay at path 214. Echo is not generally objectionable where the distant participant is located relatively close by (i.e., in the same building) but may become distracting where the echo delay is more than a few milliseconds.
To deal with echo, a traditional method is to use half-duplex operation. Here, system 200 would detect the presence of far-side audio (from the distant participant) and effectively turn off microphone 204 (which could be done by zeroing the outgoing signal) to avoid the distant party's speech being echoed back. This method has the undesirable effect of preventing both parties speaking simultaneously, and because the local participants cannot interrupt the far-side participant a conversation tends to have a perceived unnatural flow.
Certain of the more advanced conferencing systems implement an echo controller 216, which can effectively solve both of the problems of echo even where the conference proceeds in full-duplex. This is done generally by predicting the signal received at microphone 204 from the signal produced at speaker 202 and delivered over audio path 214. That predicted signal can be subtracted from the actual signal received at microphone 204, theoretically leaving only the audio of any local participants as heard by the distant participants. Predicting that signal is complex, and is affected by the frequency responses of the speaker 202 and microphone 204, and the sum of the echo paths 214 in a room at any given time. A description of a system that handles that complexity appears below, however, that particular configuration is not necessary to implement the techniques and products described and/or claimed herein.
Herein a distinction is recognized between two types of microphones. The first type of microphone is depicted in FIG. 4A, which is identified herein as a “continuous signal microphone.” In that type of microphone, as with all microphone types, a microphone element 300 is included that is a transducer converting sound waves into an electronic signal, which is usually characterized as a voltage-modulated signal for a microphone. A amplifier/receiver 302 is also included, electrically connected to element 300 which receives a signal from element 300 and controls the input impedance seen by element 300 to maintain a proper voltage range for other electronics, not shown. Now it is to be understood that other electronic components might appear between an element 300 and a receiver 302 such as pull-ups, pull-downs, amplifiers, etc., and that this figure merely illustrates in a simplified manner.
In the continuous signal microphone of FIG. 4A, element 300 is always connected to receiver 302. Thus, receiver 302 always receives an active signal from element 300. If a mute function is implemented, it is by way of a separate switch 304a or other control and input 306. Input 306 is available to conferencing system electronics to mute the microphone signal from receiver 302, which might be done, for example, by zeroing the signal or by omitting the signal from a mixer.
A continuous signal microphone is common in certain electronic devices. For example, many telephone handsets include a mute button. That button is not directly connected to the microphone element, but rather is a control input to the telephone electronics which performs the mute function. Other continuous signal microphone inputs may be provided through a GUI control, by an electronic signal, or other means.
In contrast, other types of microphones are intended to incorporate a mute function within the microphone electronics. Referring now to FIG. 4B, such a microphone is pictured with an element 300 and a receiver 302. A switch or gate 304b is also included, in this example opening the connection between element 300 and receiver 302 when in a mute position. It is to be recognized that this configuration is merely exemplary; the interruption of the signal could occur by other means, for example, by shorting a signal line. The distinguishing feature of this type of microphone is that no separate control input is provided by the microphone; rather, the muting of an input signal is done by interrupting the microphone signal. The result of this is that the conferencing electronics are not provided a definitive status of the mute function of the microphone, which microphone type is referred to as a “push-to-talk” microphone herein.
The microphone shown in FIG. 4B is but one type of push-to-talk microphone. In those microphones, switch 304b is provided near the microphone and often in the microphone housing. There, switch 304b will be configured to be normally open, such that a participant must press the button to cause the microphone to become active and not muted. For the purposes of this disclosure and the claims, the term push-to-talk includes not only microphones with normally open muting switches, but also normally closed and other switch types, and also other mute control inputs integrated into or with a microphone that do not provide a status signal to the conferencing system electronics, as will become clear from the discussion below.
The reader is referred to the following U.S. patents for background information. U.S. Pat. No. 5,933,495 to Oh describes a subband acoustic echo canceller that includes the freezing of filter coefficients on detection of near-end speech. U.S. Pat. No. 5,937,060, also to Oh, describes a residual echo suppression system in connection with an echo canceller. U.S. Pat. No. 6,990,194 to Mikesell et al. describes the use of subbanded voice activity detectors coupled to an echo cancelling circuit. All of these references are hereby incorporated by reference as background material for the description that proceeds below.